The present invention relates to a copier and other electrostatic image recording apparatuses of the type capable of transferring a toner image to any desired position on a recording medium. Also, in such an image recording apparatus, the present invention relates to a device for detecting the density of a developer which is used to produce a visible image, more specifically an implement for controlling the amount of toner supply in a development system of the kind using a two-component developer, i.e. a mixture of toner and magnetic carrier for carrying the toner.
In an electrostatic recording apparatus, it is a common practice to produce a reproduction of a desired image by a sequence of steps of forming an electrostatic latent image corresponding to the desired image on a photoconductive element or like charger carrier, depositing charged toner on the latent image, and transferring the resulting toner image to a recording medium such as a copy sheet. In a copier, for example, a photoconductive element is uniformly charged to a predetermined polarity, then it is exposed imagewise to electrostatically form a latent image, then toner is applied to the latent image, and then the resulting toner image is transferred to a coy sheet.
In a developing system of the kind using toner and carrier as stated earlier, the carrier carrying the toner is magnetically deposited on the periphery of a developing roller, which is in rotation, to form a so-called magnetic brush. The magnetic brush is brought into contact with the surface of a charge carrier so that the charged toner is electrostatically adhered to a latent image. Such a two-component developing principle is extensively applied to electrostatic image recording apparatuses of the type described. Specifically, a developing unit is provided with a reservoir in which toner and carrier are stored in a predetermined mixture ratio. A paddle wheel or the like is installed in the reservoir for agitating the toner-carrier mixture, or developer, thereby charging to toner to a predetermined polarity. Needless to mention, since only the toner is deposited on a latent image during development, the density of developer in the reservoir changes at each time of development. Microscopically, therefore, the developer density continuously changes even in the same image. So long as use is made of a large reservoir to make the amount of developer sufficiently greater than that of toner which is applied to one latent image, the microscopic change mentioned above is negligible. It is impractical, however, to use an infinitely large reservoir. For this reason, it has been customary to supply toner by detecting the developer density at a certain period. While various approaches have heretofore been proposed for such a kind of developer density control, the approach which will be described hereinafter appears most preferable.
Specifically, in a prior art electrostatic copier, a reference density pattern is provided in a certain region of a document reading surface other than that which corresponds to the leading end of an original document. At the instant when a document is to be scanned, the reference density pattern is optically scanned to form a latent image thereof on a photoconductive element, or charge carrier, the latent image being developed to become a toner image. Then, the reflectivity of the toner image is sensed by a reflection type photosensor. The level read so is compensated based on the amount of scanning light, charging level, developing bias and others and, subsequently, compared with a reference density level. If the actual level is lower than the reference level, an amount of toner matching with the drop of the level is supplemented with the developer density decided to have been lowered. This kind of developer density control can be implemented with a simple construction because a sensor for directly sensing the developer density in the reservoir is needless.
It is sometimes desired to extract or erase a part of images which are printed on an original document or even to move the position for recording it, in order to produce a so-called edited copy. An edited copy is readily attainable with an image processor of the type reading images on a document, converting them into image data, storing the image data in a mass storage, and activating, for example, a laser printer by the image data which are suitably read out of the storage as instructed by an operator. Such is impracticable, however, with an ordinary copier of the kind which directly guides a reflection from a document, which is laid on a glass platen, to a photoconductive element for thereby forming an electrostatic latent image on the photoconductive element (this kind copier will hereinafter be simply referred to as a copier). Specifically, in a copier, an operator cuts out a desired part of a document or removes the other part, then adheres it to a suitable paper (equal in size to a copy sheet in the case of 1 magnification), and then copies the paper. Alternatively, to simply bodily shift document images, the operator sets up an imaginary area on the glass platen which is equal to the size of a copy sheet and, then, positions a document based on the imaginary area. For example, assuming that an person desires to copy a document of format A4 by 1 magnification in the central area of a copy sheet of format A3, he or she assumes an area which is equal to format A3 on the glass platen and, then, sets the document such that the orthogonal lines of the document align with those of the imaginary area.
However, the procedure described above is not only inaccurate but also troublesome and, moreover, setting a document taking account of a magnification change (enlargement or reduction) is unmanageable by unskilled persons without wasting many papers for testing. Further, in a copier furnished with an automatic document feeder (ADF), the ADF is unusable. For the above reasons, there is an increasing demand for automatic processing.
Generally, in a copier, an image to be recorded on a copy sheet may be moved by shifting a toner image on a photoconductive element and a copy sheet relative to each other. Stated another way, the image can be moved by controlling the scanning timing and the sheet feeding timing.
A problem with such a scheme is that since in a prior art copier scanning depends on the size of a copy sheet, it is impossible to feed a copy sheet ahead of a toner image without entailing extra scanning and, therefore, without lowering the copying speed.
Further, a problem is also brought about when the developer density is sensed. In a prior art copier, the reference pattern located in the particular region of a document reading surface as previously stated is scanned to form its latent image on a photoconductive element. Specifically, assuming that the distance between the leading end of a document and the reference pattern is l and the ratio of magnficiation change is N, the latent image of the reference pattern is formed in a position ahead of the latent image corresponding to the leading end of the document by a distance of Nl. This sometimes causes the toner image of the reference pattern to be recorded on a copy sheet, i.e., adequate control over the developer density is impracticable.